This invention relates to a method and an apparatus for producing a transparent conductive film and in particular to a method and an apparatus for producing an In-O, Sn-O, Zn-O, Cd-Sn-O or Cd-In-O based transparent conductive film used as an electrode for such devices as a liquid crystal display element, a solar battery or the like.
There are conventionally known processes for producing this kind of transparent conductive film such as coating, vacuum deposition, gas-phase reaction methods, as well as sputtering methods that include the DC or RF double-pole sputtering method and the DC or RF magnetron sputtering method. Among these producing processes, the sputtering methods are superior to the other producing methods in that a transparent conductive film of relatively low electrical resistance is easily obtained and that this transparent conductive film can be formed uniformly and controllably on a large-sized substrate with good reproducibility or repeatability. Among the different sputtering methods, the magnetron sputtering method which utilizes plasma confinement by a magnetic field on a surface of a target is generally used because it has a high film-forming speed and is better for mass production. Further, for an electric power supply for generating electric discharging, a DC power supply is normally used because it is superior in cost, uniformity of electric discharging, film-forming speed and the like.
When producing a transparent conductive film using one of the sputtering methods, the temperature of the substrate and the partial pressure of oxygen are factors which affect the electrical resistivity of the transparent conductive film. As regards the substrate temperature, it is known that the higher the substrate temperature, the lower the electrical resistivity of the resulting film. On the other hand, as regards the partial pressure of oxygen, it is known that with a lower oxygen partial pressure, the density of carriers is higher, and the mobility is lower because there are many vacancies of oxygen which would normally act as donors. With a higher oxygen partial pressure, the density of carriers is lower and the mobility is higher. Thus, there is an optimum partial pressure of oxygen that will result in an electrical resistivity of a minimum value from an even balance of the density and the mobility. Thus, it was a practice in the prior art sputtering methods to produce a transparent conductive film having a lower electrical resistivity wherein parameters of the substrate temperature and the partial pressure of oxygen were controlled.
However, it is required in present-day display elements that the transparent electrode have a lower electrical resistance because of enlargements in the display screen size. The prior art sputtering methods can no longer meet the requirements. Especially, in a display element of a simple matrix drive system, a transparent conductive film is used in the scanning signal electrode. If the electric resistance of the transparent conductive film is high, the image quality deteriorates. Therefore, the electrical resistance of the transparent conductive film must be low. In addition, in full color STN display elements which have attracted attention recently, a transparent conductive film is normally formed on an organic color filter. The temperature of forming the transparent conductive film is limited to about 160.degree. to 200.degree. C. due to the heat-resisting temperature of the filter, and thus forming the transparent conductive film at a lower temperature becomes necessary.
The inventors of this invention already found out, as disclosed in Japanese Patent Application No. 150086/1989 (Japanese Published Unexamined Patent Application No. 232358/1990), that the factors largely affecting the electrical resistivity in producing a transparent conductive film by the sputtering methods are the discharging voltage during sputtering, along with the above-described substrate temperature and the partial pressure of oxygen. Consequently, a method of producing a transparent conductive film was proposed in which a transparent conductive film of lower resistance can be obtained by sputtering at a low sputtering voltage. This method is based on the following idea. When a transparent oxide conductive film is formed by a sputtering method, anions of oxygen are generated by the ionization of oxygen in the introduced oxygen gas or in the target composition. These anions smash into the substrate, thereby causing micro-damage to the transparent conductive film that is being formed and consequently deteriorating the film characteristics such as resistance or the like. Since these anions are accelerated by an electric field generated by the negative electric potential of the target, the degree to which the formed transparent conductive film deteriorates is proportional to the energy of the anions (i.e., the negative electric potential of the target).
Here, the target during sputtering is understood to have a negative electric potential and an absolute value thereof is called a sputtering voltage or a discharging voltage.
In the producing method of the above-described patent application, the intensity of a magnetic field on the surface of the target was increased and the density of plasma produced by magnetron discharging was increased, resulting in a decrease or lowering in the electric discharging voltage. By lowering the sputtering voltage down to about 250V in the above-described method while that in the conventional DC magnetron sputtering method is about 400V (the target electric potential being -400V), it was possible to largely decrease the electrical resistivity of the formed transparent conductive films which were produced at various substrate temperatures ranging from room temperature to 400.degree. C. or above. The electrical resistivity of the transparent conductive film linearly dropped relative to the sputtering voltage within the sputtering voltage range of 400 to 250V. It was expected that the electrical resistivity would further lower also in the range of sputtering voltage of 250V or below.
However, in the above-described method of controlling the sputtering voltage by varying only the intensity of the magnetic field, it is difficult to lower the sputtering voltage below 250V, as can be seen form FIG. 1. FIG. 1 shows the relationship between the intensity of the magnetic field on the surface of the target and the discharging voltage during sputtering. Namely, the sputtering voltage effectively decreases to about 1000 Oe with the increase in the intensity of the magnetic field. Above 1000 Oe, however, the decrease in the sputtering voltage is almost saturated, and the sputtering voltage reaches about 250V at 1600 Oe. As a consequence, even through the intensity of the magnetic field is further increased, no further decrease in the sputtering voltage can be expected.
Therefore, by eliminating the problems associated with the conventional methods, the present invention can provide a method of and an apparatus for producing a transparent conductive film with a still lower electrical resistance.